Model Evaluation of Secondary Chemistry due to Disinfection of Indoor Air with Germicidal Ultraviolet Lamps

Air disinfection using germicidal ultraviolet light (GUV) has received increasing attention during the COVID-19 pandemic. GUV uses UVC lamps to inactivate microorganisms, but it also initiates photochemistry in air. However, GUV’s indoor-air-quality impact has not been investigated in detail. Here, we model the chemistry initiated by GUV at 254 (“GUV254”) or 222 nm (“GUV222”) in a typical indoor setting for different ventilation levels. Our analysis shows that GUV254, usually installed in the upper room, can significantly photolyze O3, generating OH radicals that oxidize indoor volatile organic compounds (VOCs) into more oxidized VOCs. Secondary organic aerosol (SOA) is also formed as a VOC-oxidation product. GUV254-induced SOA formation is of the order of 0.1–1 μg/m3 for the cases studied here. GUV222 (described by some as harmless to humans and thus applicable for the whole room) with the same effective virus-removal rate makes a smaller indoor-air-quality impact at mid-to-high ventilation rates. This is mainly because of the lower UV irradiance needed and also less efficient OH-generating O3 photolysis than GUV254. GUV222 has a higher impact than GUV254 under poor ventilation due to a small but significant photochemical production of O3 at 222 nm, which does not occur with GUV254

Organic Aerosols in the Earth’s Atmosphere

Organic Aerosols in the Earth’s Atmospher

Correction to “Model Evaluation of Secondary Chemistry due to Disinfection of Indoor Air with Germicidal Ultraviolet Lamps”

Correction to “Model Evaluation of Secondary Chemistry due to Disinfection of Indoor Air with Germicidal Ultraviolet Lamps

Effect of Vaporizer Temperature on Ambient Non-Refractory Submicron Aerosol Composition and Mass Spectra Measured by the Aerosol Mass Spectrometer

<div><p>Aerodyne Aerosol Mass Spectrometers (AMS) are routinely operated with a constant vaporizer temperature (<i>T</i>_vap) of 600°C in order to facilitate quantitative detection of non-refractory submicron (NR-PM₁) species. By analogy with other thermal desorption instruments, systematically varying <i>T</i>_vap may provide additional information regarding NR-PM₁ chemical composition and relative volatility, and was explored during two ambient studies. The performance of the AMS generally and the functional integrity of the vaporizer were not negatively impacted during vaporizer temperature cycling (VTC) periods. NR-PM₁ species signals change substantially as <i>T</i>_vap decreases with that change being consistent with previous relative volatility measurements: large decreases in lower volatility components (e.g., sulfate, organic aerosol [OA]) with little, if any, decrease in higher volatility components (e.g., nitrate, ammonium) as <i>T</i>_vap decreases. At <i>T</i>_vap < 600°C, slower evaporation was observed as a shift in particle time-of-flight distributions and an increase in “particle beam blocked” (background) concentrations. Some chemically reduced (i.e., C<i>_x</i>H<i>_y</i>⁺) OA ions at higher <i>m/z</i> are enhanced at lower <i>T</i>_vap, indicating that this method may improve the analysis of some chemically reduced OA systems. The OA spectra changes dramatically with <i>T</i>_vap; however, the observed trends cannot easily be interpreted to derive volatility information. Reducing <i>T</i>_vap increases the relative O:C and CO₂⁺, contrary to what is expected from measured volatility. This is interpreted as continuing decomposition of low volatility species that decreases more slowly (as <i>T</i>_vap decreases) than does the evaporation of reduced species. The reactive vaporizer surface and the inability to reach <i>T</i>_vap much below 200°C of the standard AMS limit the ability of this method to study the volatility of oxidized OA species.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

Elemental Analysis of Organic Species with Electron Ionization High-Resolution Mass Spectrometry

We present a new elemental analysis (EA) technique for organic species (CHNO) that allows fast on-line analysis (10 s) and reduces the required sample size to ∼1 ng, ∼6 orders of magnitude less than standard techniques. The composition of the analyzed samples is approximated by the average elemental composition of the ions from high-resolution electron ionization (EI) mass spectra. EA of organic species can be performed on organic/inorganic mixtures. Elemental ratios for the total organic mass, such as oxygen/carbon (O/C), hydrogen/carbon (H/C), and nitrogen/carbon (N/C), in addition to the organic mass to organic carbon ratio (OM/OC), can be determined. As deviations between the molecular and the ionic composition can appear due to chemical influences on the ion fragmentation processes, the method was evaluated and calibrated using spectra from 20 compounds from the NIST database and from 35 laboratory standards sampled with the high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The analysis of AMS (NIST) spectra indicates that quantification of O/C is possible with an error (average absolute value of the relative error) of 30% (17%) for individual species. Precision is much better than accuracy at ±5% in the absence of air for AMS data. AMS OM/OC has an average error of 5%. Additional calibration is recommended for types of species very different from those analyzed here. EA was applied to organic mixtures and ambient aerosols (sampled at 20 s from aircraft). The technique is also applicable to other EI-HRMS measurements such as direct injection MS

Quantification of Gas-Wall Partitioning in Teflon Environmental Chambers Using Rapid Bursts of Low-Volatility Oxidized Species Generated in Situ

Partitioning of gas-phase organic compounds to the walls of Teflon environmental chambers is a recently reported phenomenon than can affect the yields of reaction products and secondary organic aerosol (SOA) measured in laboratory experiments. Reported time scales for reaching gas-wall partitioning (GWP) equilibrium (τGWE) differ by up to 3 orders of magnitude, however, leading to predicted effects that vary from substantial to negligible. A new technique is demonstrated here in which semi- and low-volatility oxidized organic compounds (saturation concentration c* < 100 μg m–3) were photochemically generated in rapid bursts in situ in an 8 m3 environmental chamber, and then their decay in the absence of aerosol was measured using a high-resolution chemical ionization mass spectrometer (CIMS) equipped with an “inlet-less” NO3– ion source. Measured τGWE were 7–13 min (rel. std. dev. 33%) for all compounds. The fraction of each compound that partitioned to the walls at equilibrium follows absorptive partitioning theory with an equivalent wall mass concentration in the range 0.3–10 mg m–3. Measurements using a CIMS equipped with a standard ion–molecule reaction region showed large biases due to the contact of compounds with walls. On the basis of these results, a set of parameters is proposed for modeling GWP in chamber experiments

A Case Study of Urban Particle Acidity and Its Influence on Secondary Organic Aerosol

Size-resolved indicators of aerosol acidity, including H+ ion concentrations (H+Aer) and the ratio of stoichiometric neutralization are evaluated in submicrometer aerosols using highly time-resolved aerosol mass spectrometer (AMS) data from Pittsburgh. The pH and ionic strength within the aqueous particle phase are also estimated using the Aerosol Inorganics Model (AIM). Different mechanisms that contribute to the presence of acidic particles in Pittsburgh are discussed. The largest H+Aer loadings and lowest levels of stoichiometric neutralization were detected when PM1 loadings were high and dominated by SO42-. The average size distribution of H+Aer loading shows an accumulation mode at Dva ≈ 600 nm and an enhanced smaller mode that centers at Dva ≈ 200 nm and tails into smaller sizes. The acidity in the accumulation mode particles suggests that there is generally not enough gas-phase NH3 available on a regional scale to completely neutralize sulfate in Pittsburgh. The lack of stoichiometric neutralization in the 200 nm mode particles is likely caused by the relatively slow mixing of gas-phase NH3 into SO2-rich plumes containing younger particles. We examined the influence of particle acidity on secondary organic aerosol (SOA) formation by comparing the mass concentrations and size distributions of oxygenated organic aerosol (OOAsurrogate for SOA in Pittsburgh) during periods when particles are, on average, acidic to those when particles are bulk neutralized. The average mass concentration of OOA during the acidic periods (3.1 ± 1.7 μg m-3) is higher than that during the neutralized periods (2.5 ± 1.3 μg m-3). Possible reasons for this enhancement include increased condensation of SOA species, acid-catalyzed SOA formation, and/or differences in air mass transport and history. However, even if the entire enhancement (∼0.6 μg m-3) can be attributed to acid catalysis, the upper-bound increase of SOA mass in acidic particles is ∼25%, an enhancement that is much more moderate than the multifold increases in SOA mass observed during some lab studies and inferred in SO2-rich industrial plumes. In addition, the mass spectra of OOA from these two periods are almost identical with no discernible increase in relative signal intensity at larger m/z's (>200 amu), suggesting that the chemical nature of SOA is similar during acidic and neutralized periods and that there is no significant enhancement of SOA oligomer formation during acidic particle periods in Pittsburgh

Elemental Analysis of Complex Organic Aerosol Using Isotopic Labeling and Unit-Resolution Mass Spectrometry

Elemental analysis of unit-mass resolution (UMR) mass spectra is limited by the amount of information available to definitively elucidate the molecular formula of a molecule ionized by electron impact. The problem is compounded when a mixture of organic molecules (such as those found in organic aerosols) is analyzed without the benefit of prior separation. For this reason, quadrupole mass spectrometry is not usually suited to the elemental analysis of organic mixtures. Here, we present a mathematical method for the elemental analysis of UMR mass spectra of a complex organic aerosol through the use of isotopic labeling. Quadrupole aerosol mass spectrometry was used to obtain UMR data of ¹³C-labeled and unlabeled aerosol generated by far ultraviolet (FUV) photochemistry of gas mixtures containing 0.1% of either CH₄ or ¹³CH₄ in N₂. In this method, the differences in the positions of ion groups in the resulting spectra are used to estimate the mass fraction of carbon in the aerosol, and estimation of the remaining elements follows. Analysis of the UMR data yields an elemental composition of 63 ± 7% C, 8 ± 1% H, and 29 ± 7% N by mass. Unlabeled aerosols formed under the same conditions are found by high-resolution time-of-flight aerosol mass spectrometry to have an elemental composition of 63 ± 3% C, 8 ± 1% H, 20 ± 4% N, and 9 ± 3% O by mass, in good agreement with the UMR method. This favorable comparison verifies the method, which expands the UMR mass spectrometry toolkit

Impacts of Aerosol Aging on Laser Desorption/Ionization in Single-Particle Mass Spectrometers

<div><p>Single-particle mass spectrometry (SPMS) has been widely used for characterizing the chemical mixing state of ambient aerosol particles. However, processes occurring during particle ablation and ionization can influence the mass spectra produced by these instruments. These effects remain poorly characterized for complex atmospheric particles. During the 2005 Study of Organic Aerosols in Riverside (SOAR), a thermodenuder was used to evaporate the more volatile aerosol species in sequential temperature steps up to 230°C; the residual aerosol particles were sampled by an aerosol mass spectrometer (AMS) and a single-particle aerosol time-of-flight mass spectrometer (ATOFMS). Removal of the secondary species (e.g., ammonium nitrate/sulfate) through heating permitted assessment of the change in ionization patterns as the composition changed for a given particle type. It was observed that a coating of secondary species can reduce the ionization efficiency by changing the degree of laser absorption or particle ablation, which significantly impacted the measured ion peak areas. Nonvolatile aerosol components were used as pseudo-internal standards (or “reference components”) to correct for this LDI effect. Such corrected ATOFMS ion peak areas correlated well with the AMS measurements of the same species up to 142°C. This work demonstrates the potential to accurately relate SPMS peak areas to the mass of specific aerosol components.</p><p>Copyright 2014 American Association for Aerosol Research</p></div

Liquid Water: Ubiquitous Contributor to Aerosol Mass

Aerosol liquid water (ALW) is a ubiquitous component of atmospheric aerosol and influences particle chemistry, visibility, human health, and regional climate. The global abundance and spatial patterns in ALW mass concentrations and its fractional contribution to total particle mass are not routinely documented. We estimate lower-bound ALW mass concentrations at locations and time periods of aerosol mass spectrometer (AMS) field campaigns using speciated ion measurements, meteorology from the Climate Forecast System Reanalysis, and thermodynamic predictions from ISORROPIA version 2.1. The contribution of water by organic compounds is estimated using κ-Kohler theory. Field campaign-specific patterns suggest that ALW mass is largest in urban and urban downwind areas, and that of growth factors is largest in rural areas. The highest average ALW mass concentration is estimated for the AMS study in Beijing and the highest mass fraction for rural Hyytiala. A more robust understanding of ALW is critical for developing and improving models that predict air quality and climate